diff --git a/src/libutil/archive.cc b/src/libutil/archive.cc
index 465df2073..712ea51c7 100644
--- a/src/libutil/archive.cc
+++ b/src/libutil/archive.cc
@@ -140,7 +140,7 @@ static void parseContents(ParseSink & sink, Source & source, const Path & path)
     sink.preallocateContents(size);
 
     uint64_t left = size;
-    std::vector<char> buf(65536);
+    std::array<char, 65536> buf;
 
     while (left) {
         checkInterrupt();
diff --git a/src/libutil/file-system.cc b/src/libutil/file-system.cc
index c96effff9..4cac35ace 100644
--- a/src/libutil/file-system.cc
+++ b/src/libutil/file-system.cc
@@ -307,7 +307,7 @@ void writeFile(const Path & path, Source & source, mode_t mode, bool sync)
     if (!fd)
         throw SysError("opening file '%1%'", path);
 
-    std::vector<char> buf(64 * 1024);
+    std::array<char, 64 * 1024> buf;
 
     try {
         while (true) {
diff --git a/src/libutil/serialise.cc b/src/libutil/serialise.cc
index f465bd0de..76b378e18 100644
--- a/src/libutil/serialise.cc
+++ b/src/libutil/serialise.cc
@@ -82,7 +82,7 @@ void Source::operator () (std::string_view data)
 void Source::drainInto(Sink & sink)
 {
     std::string s;
-    std::vector<char> buf(8192);
+    std::array<char, 8192> buf;
     while (true) {
         size_t n;
         try {